Energy harvesting involves capturing ambient or background energy from the environment and converting it into usable electrical energy. This is typically stored and then used to power electrical loads such as electronic devices. In energy harvesting, energy transducers are used to bring the ambient energy into the electrical domain. For example, a thermoelectric generator makes use of a temperature difference, a piezoelectric transducer generates a voltage from mechanical strain, a photovoltaic cell converts light into electricity, and an electrodynamic transducer converts mechanical energy such as vibrations into electrical energy. The electrical energy from all such transducers can be stored in any suitable storage device, such as a capacitor, a super-capacitor, or a rechargeable battery, or can be stored in an appropriate group or combination of such storage devices.
The outputs of the various types of energy transducers have a wide range of different electrical characteristics. FIG. 1 of the accompanying drawings illustrates this for various types, by way of example. The characteristics described with reference to FIG. 1 are exemplary, and the values given are approximate. A thermoelectric generator (TEG) has a resistive output impedance, and generates a steady flow (a trickle) of direct-current electricity (DC) of either polarity, in a current range of 1 uA to 100 mA over a voltage range of 20 mV to 2V. A photovoltaic transducer (PV) also has a resistive output impedance, and generates a steady flow of DC electricity, in a current range of 1 uA to 100 mA over a voltage range of 400 mV to 4V, typically with a single polarity. An electrodynamic transducer (ED) has a mainly inductive output impedance, and typically generates energy in bursts or pulses, either as DC electricity of either polarity, or as alternating-current electricity (AC). Its output typically lies in a current range of 100 uA to 80 mA over a voltage range of 2V to 20V. A piezoelectric transducer (PZ) has a mainly capacitive output impedance, and generates AC electricity in bursts, in a current range of 1 uA to 100 uA over a voltage range of 5V to 50V.
As will be appreciated from FIG. 1, the wide range of power levels and output impedances of energy harvesting transducers presents a significant engineering challenge when interfacing to such devices. As a result, many of the existing energy-harvesting solutions are transducer-specific.
To extract the maximum amount of energy from a transducer, an energy converter that presents the transducer with a matched electrical impedance is used. In some cases, the maximum power point varies as the prevailing environmental conditions change. Existing energy converters, and power management units (PMUs) including such converters, for use in an energy harvesting application tend to focus on respective particular types of energy transducer, and provide targeted solutions for use with those particular types only. Operation with other types of energy transducer is often not possible, or involves accepting significant performance compromises.
Existing energy converters and PMUs typically have a single input for connection to a single energy transducer. Solutions with multiple inputs generally require that all the transducers are of a similar type, or have similar characteristics.
Some known energy converters claim operation with multiple transducer types but, in reality, provide an optimal solution with only a single transducer type. With other types of energy transducer, such converters are compromised. For example: Bridge rectifiers may be appropriate for PZ transducers, but they are inefficient or impractical for use with low-voltage DC sources; PZ transducers are relatively high voltage sources, and a corresponding voltage from PV would require many PV cells in series. None of the available energy converters and PMUs meet the need for harvesting energy from a variety of sources.
It is, therefore, desirable to provide energy converters, PMUs and associated systems and methods that address the drawbacks of existing products and solutions, and to provide a single PMU which properly supports multiple energy-harvesting transducers of different types and different power levels.